This invention relates to an antilock control system and method for controlling vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical slip value is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock braking control system (ABS) seeks to operate wheel slip at or near the critical slip value.
In one known ABS, the brake pressure is modulated by means of a DC torque motor driving a piston in a cylinder whose volume is modulated to control the hydraulic pressure at the wheel brake. The motor is controlled to position the piston at an initial, fully extended home position at which a check valve is unseated to couple the brake system master cylinder to the wheel brake to allow normal braking. When antilock brake pressure modulation is required, the motor retracts the piston (which allows the check valve to close to isolate the master cylinder from the wheel brake) to reduce brake pressure at the wheel brake and thereafter modulates the piston position to provide pressure control for antilock braking. When antilock braking is no longer required, the motor returns the piston to its extended home position. Typically, in these prior systems, the hydraulic pressure at the wheel brake is established based upon the relationship between motor current, motor torque and the hydraulic brake pressure acting on the head of the piston (wheel brake pressure). Motor current as a representation of wheel brake pressure then becomes the controlled parameter to establish a desired braking condition via the brake pressure/motor current relationship.
During ABS controlled braking, when an incipient wheel lock condition is sensed, an ABS cycle is initiated beginning with a pressure release phase wherein the value of motor current at the time the incipient wheel lock condition was sensed is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, a pressure apply phase is initiated in which the motor current is controlled to extend the piston to reapply brake pressure. During the apply phase, the motor current is ramped to ramp the brake pressure at a controlled rate in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the ABS cycle is repeated.
In the pressure apply phase of this ABS cycle, it is desirable to rapidly restore the proper amount of brake pressure when a recovery from an incipient wheel lock condition is first sensed prior to a more gradual ramping of the pressure. This is typically accomplished by an "apply bump" wherein brake pressure is increased at a high rate for a period of time, such as by commanding maximum current to the motor in the above described known system. Thereafter, the brake pressure is ramped at the more gradual rate until an incipient wheel lock condition is again sensed thereby initiating another ABS cycle.
It is desirable to begin ramping the brake pressure from an initial apply pressure that is equal to the brake pressure at the end of the apply bump. However, in the foregoing form of motor driven pressure modulator, when the brake pressure load on the motor is less than the motor torque, the motor accelerates while extending the piston to increase pressure. When this condition exists, the motor current is not a true indicator of brake pressure due to the motor dynamics, such as the motor back EMF as the motor accelerates. One method of determining the motor current that results in a brake pressure existing at the end of the current bump estimates the pressure based on the motor current at the end of the prior apply phase at the time an incipient wheel lock condition was first sensed and the time of release of brake pressure.